The present invention relates to a method and an apparatus for thickening fiber suspension. In particular, this invention is directed to draining water out of pulp which is made to thicken in a closed space equipped with a fine-perforated or a fineslotted screen surface. This is realized by maintaining a pressure difference necessary for thickening over the screen surfaces.
Fiber material, in particular cellulose and wood fiber pulp, is usually treated in the form of a dilute suspension. For instance, screening with perforated or slotted screens is carried out at a consistency of 1% to 3%. After screening, the fiber suspension must, due to various reasons, be thickened to a higher consistency. Often the consistency is raised to a range of 10% to 15%, e.g. for storage or refining of reject.
The pulp and paper industry knows various suction filters which have been given much attention in the literature of the field. Thickening with suction filters is achieved by sucking water out of the pulp through a filter fabric. There are also a number of other ideas or apparatuses being developed, which can be used to extract large quantities of fluid from the pulp suspension. What all these have in common is the fact that they are complex in structure and include moving parts. An example of such an apparatus is described in U.S. Pat. No. 3,455,821, which utilizes vibrations to clean the screen surface.
U.S. Pat. No. 3,870,638 describes cleaning of screen surface by means of back-blowing and pressure pulses only. The problem is, however, that it is difficult to time the back-blowing and pressure pulses accurately enough to clean the screen and make the concentrated pulp flow on and the dilute pulp fill the thickening zone of the screen area in one swift motion. This apparatus also has the characteristic feature that the concentrated pulp stays immobile in respect to the screen surface during the thickening stage.
The above-mentioned apparatuses usually give a consistency of 10% to 15%. However, a higher consistency than that is often required. According to trade literature, roll presses or pressure screws are generally used to reach higher consistencies.
For roll presses, the pulp is spread out as web and led through two perforated rolls. The rolls form a press nip, which squeezes water out of the pulp. After the pressing process, the pulp drops on a receiver screw which takes it to the next treatment stage. A disadvantage in such an apparatus is its complex construction and the mixing of air with the pulp during after-press stages.
In the pressure screw, water is removed from the pulp by forcing the pulp into a converging space, which is equipped with a screen surface. This forces the water to be drained through the screen surface, which raises the consistency. The problem with this kind of an apparatus is its complex structure and the high risk of clogging of the screen surface perforations.
So-called tube filters are known to be a simpler form of thickening/filtering structures. An example of these is a method and an apparatus presented in U.S. Pat. No. 3,794,179 for thickening of fluid containing solid matter. The operation of the apparatus relies on thickening of solid material on a screen surface at the initial stage. At the next stage, the concentrated matter is washed with a strong flow. Continuous flushing is an alternative cleaning method. Whichever method is used, there remains the problem that the concentrated matter is diluted through the use of washing liquid. Thus, the apparatus is more applicable to production of filtrate lacking solid matter than to thickening of pulp suspension.
Another example of a tubular filter is an apparatus according to U.S. Pat. No. 3,674,154, where the problem is clogging of the screen surface. This can be avoided at least partly by taking advantage of the widening of the cross sectional area of flow, which according to the same disclosure, keeps particles in the middle of the flow due to their slowness and enables the lighter fluid to move to the walls and out through the holes in the screen surface. Such an apparatus is useful when separating relatively large particles out of a fast-flowing dilute fluid. However, the apparatus is not suitable to thickening of pulp suspension, because the fibers are not loose in the suspension but form a three-dimensional fiber network, which has the tendency of expanding due to the bending force of fibers. Therefore, the fibers do not remain in the middle of the flow. Another drawback for the use of an apparatus according to this US disclosure is the fact that the pulp flow proceeds as a plug flow, which due to its relatively stiff nature prevents the flow from behaving the way fluid matter usually does at the constriction and expansion points.
A third solution which resembles the previous ones is an apparatus according to US Patent Disclosure 4,421,646, which is a tubular filter including several layered screen plates or corresponding means disposed on top of each other. Between these surfaces there is material which performs the fine filtration. This solution is, as mentioned in the disclosure, meant for screening of water or corresponding substance and not for thickening of suspension. Furthermore, the apparatus would not be applicable to thickening of fiber suspension, since the slots in the screen plates would actually collect fibers and on the other hand the fine filtering material between the plates would immediately be clogged by the fine material that the pulp suspension contains.
A fourth example of thickening apparatuses is a tubular thickener according to U.S. Pat. No. 2,998,064, which is designed for removing liquid from chips-liquid suspension. The walls of the thickener consist of ribs parallel to the flow between which ribs there are slots so small that wooden chips can not penetrate through them. In addition, the diameter of the tube inlet corresponds to the inner diameter of the thickener and the diameter of the tube outlet corresponds to the outer diameter of the thickener, so even the chips that partly have found their way into the slots are partly carried away by the flow and they do not clog the thickener from the discharge end backwards. Structurally, however, this kind of a thickener is difficult to realize, since the wall of the thickener consists of loose bars whose firm connection to each other to provide equal slots demands great accuracy and care.